Machines may include one or more hydraulic circuits for operation of the machine, including a hydraulic transmission for providing propulsion for the machine. The hydraulic transmission may include one or more hydraulic circuits including a hydraulic pump and a hydraulic motor. For example, some machines may include a variable displacement hydraulic pump and/or a variable displacement hydraulic motor. The hydraulic pump may be driven by a prime mover. By varying the displacement and/or speed of the hydraulic pump, the amount of fluid pumped to the hydraulic motor may be controlled. Fluid pumped through the hydraulic motor causes it to spin an output shaft coupled to one or more ground engaging members, such as wheels and/or tracks. By varying the ratio between the displacement of the hydraulic pump and the displacement of the hydraulic motor, the speed at which the hydraulic motor drives the output shaft may be controlled.
Under certain operational conditions, pressure may build in the hydraulic circuit to a level higher than desired, and thus, it may be desirable to regulate the pressure in the hydraulic circuit. For example, when the movement of the machine is inhibited by an external resistance, pressure may build within the hydraulic circuit that drives a ground engaging member, for example, when the machine pushes against a large pile of earth. This may result in travel of the machine being slowed or stopped. The slowing or stopping of the machine may cause the ground engaging member to also be slowed or stopped, which, in turn, slows or stops the hydraulic motor that turns the ground engaging member. As the hydraulic motor is slowed or stopped, the flow of fluid through the hydraulic motor may be substantially inhibited, while the flow of fluid supplied from the hydraulic pump may continue at substantially the same flow rate. As a result, the pressure in the circuit may increase beyond desired levels, which may be related to machine design limits and/or the physical characteristics of the fluid in the hydraulic circuit.
Systems have been developed that reduce pressure in hydraulic circuits. For example, U.S. Pat. No. 5,941,689, issued on Aug. 29, 1999, to Skirde (the '689 patent), discloses a control system for a variable pump and a process to control the variable pump. A circuit includes the variable pump, which is connected to a hydraulic motor by working lines having check valves in the circuit to allow oil flow from a charge pump to the circuit. The circuit also includes high pressure relief valves to allow oil flow out of the working lines to limit maximum pressure. A temperature sensor is placed between the high pressure relief valves and the check valves. According to the system and method of the '689 patent, a signal is processed in an electronic control to control the displacement of a variable pump, depending on the temperature of the fluid flowing across the high pressure relief valves.
Although the system and method disclosed in the '689 patent may serve to limit the maximum pressure in the circuit, the system and method may result in unsatisfactory performance of the circuit. For example, the pressure in the circuit may be reduced in a manner that unnecessarily reduces the circuit's responsiveness to operator commands. Therefore, it may be desirable to provide a system and method for controlling a hydraulic system in manner that results in desirable operation of a machine while maintaining the pressure in the hydraulic circuit within desired levels.